Word for the "strength" of an adjective

Solution 1:

The technical word for this is "Word Spectrum." As seen in the Dictionary App on OSX (more specifically only shown in the Oxford's American Writer Thesaurus). Another thing you may want to check out is "Aristotle's Golden Mean." Not the same, but in the spectrum category.

Solution 2:

I've found a couple of papers, which use the term "Adjective Polarity" to describe the position of each related adjective within a numerical scale.

"polarity" refers to the way the scale includes both positive and negative values, which cunningly allows antonyms and synonyms to be placed within the same series.

From what I understand, a word's polarity value is subjective, and dependant upon the classification technique chosen. Here's an example of how we may choose to assign polarity values for adjectives of "quality":

-2  terrible
-1  bad
 0  acceptable
+1  good
+2  great

Here are the papers for reference:

Agarwal & Bhattacharyya
Williams & Anand